1. Field of the Invention
The present invention relates generally to a shut-off valve for a liquid fuel, disposed in a fuel vapor path extending between the upper hollow space of a fuel tank for a vehicle and a canister for storably receiving the fuel vapor generated in the fuel tank. The shut-off valve serves to prevent the liquid fuel from flowing into the canister at the time of an occurrence of a vehicular accident or emergency such as abnormal inclination of the vehicle, abnormal turning movement or the like. More particularly, the present invention relates to improvement of a shut-off valve of the foregoing type.
2. Description of the Prior Art
To facilitate understanding of the present invention, two typical conventional shut-off valves of the foregoing type will be described below with reference to FIG. 6 and FIG. 7.
FIG. 6 shows a sectional elevational view of the structure of a conventional shut-off valve for a liquid fuel as disclosed in Japanese Patent laid-open Publication NO. 2-112658. In the drawing, reference numeral 1 designates a valve housing. A plurality of fuel vapor inflow holes 2 are formed through the bottom wall of the valve housing 1. The fuel vapor inflow holes 2 are open to the upper space of a fuel tank (not shown) located above the surface level of a liquid fuel in the fuel tank. Reference numeral 3 designates a fuel vapor outflow hole. The fuel vapor outflow hole 3 communicates with a canister (not shown) in which the fuel vapor generated in the fuel tank is storably received.
Reference numeral 4 designates a float, and reference numeral 6 designates a valve disc. When a liquid fuel from the fuel tank flows into the valve housing 1 through the fuel vapor inflow holes 2, due to abnormal inclination of a vehicle or abnormal turning movement, the float 4 is raised up by its buoyancy, causing a through hole 6 formed through the central part of the valve disc 5 to be sealably closed by the upper surface of the float 4 as shown in FIG. 6. Subsequently, the valve disc 5 is raised up until the upper surface of the valve disc 5 comes into sealing contact with the annular lower end edge of the top of the housing around the fuel vapor outflow hole 3, to thereby prevent any inflow of the liquid fuel into the canister through the fuel vapor outflow hole 3. When the vehicle is restored to the normal running state and the liquid fuel is returned from the valve housing 1 to the fuel tank through the fuel vapor inflow holes 2, the float 4 will be lowered and the force of gravity on the valve disc 5 and the float 4 will overcome the force urging the float 4 into sealable contact with the annular housing top around the through hole 6 of the valve plug 5, the latter force being the vapor pressure prevailing in the fuel tank. Thus, the float 6 downwardly separates from the through hole 6, resulting in the opening of the through hole 6 of the valve disc 5, whereby the fuel vapor pressure is reduced or relieved. In other words, the valve plug 5 separates from the fuel vapor outflow hole 3 by the force of gravity on the valve disc 5 itself. The valve disc 5 is further forced away from the fuel vapor outflow hole 6 by the force of gravity acting on the float 4 due to engagement of a flange 7 of the float 4 with an annular shoulder 8 of the valve disc 5. Once the valve disc 5 is parted away from the fuel vapor outflow hole 6, the fuel vapor generated in the fuel tank is introduced into the canister via the fuel vapor inflow holes 2 and the fuel vapor outflow hole 3. Incidentally, reference numeral 9 designates a coil spring which assists the valve closing function of the float 4. While the float 4 is immersed in the liquid fuel, the valve disc 5 is held in the closed state by the buoyant force of the float 4 and the resilient force of the coil spring 9. Even when the vehicle is inclined or it is turned upside down (through an angle of 180 degrees) due to a vehicular accident or emergency, the valve disc 5 is likewise held in the closed state due to a component of the gravity acting on the float 4, corresponding to the angle of inclination, and to the resilient force of the coil spring 9.
FIG. 7 shows a sectional elevational view of the structure of another conventional shut-off valve for a liquid fuel as disclosed in Japanese Utility Model Laid-Open Publication NO. 4-134733 filed by an inventor of the present invention. In the drawing, reference numeral 1 designates a valve housing, reference numeral 2 designates a plurality of fuel vapor inflow holes, reference numeral 3 designates a fuel vapor outflow hole, reference numeral 4 designates a float 4, reference numeral 5 designates a valve disc, and reference numeral 6 designates a through hole formed through the valve disc 5. The upper part of the valve disc 5 has a semispherical contour for a reliable, sealable contact with the annular area of the housing defining the lower end of the fuel vapor outflow hole 3. In addition, the upper end of a rod-shaped protuberance 42, integrated with a main body 41 of the float 4, likewise has a semispherical contour for reliably sealing the lower end of the through hole 6. With this construction, even in the case that the valve disc 5 or the float 4 is brought into sealing contact with the lower end of the fuel vapor outflow hole 3 or the through hole 5, while inclined relative to the fuel vapor outflow hole 3 or the through hole 6 for some reason, liquid fuel shut-off can be reliably achieved. In addition, engagement of the float 4 with the valve disc 5 during downward displacement of the float 4 is attained by engagement of a plurality of engagement pawls 43 formed inside of a float casing 41 with a flange portion 51 formed around the outer periphery of the valve disc 5 at the lower end of the latter.
Reference numeral 11 designates a plurality of guide ribs formed around the inner wall surface of the valve housing 1, and reference numeral 12 designates a flange portion formed on the exterior of the valve housing 1. That portion of the valve housing 1 located below the flange portion 12 is normally located within a fuel tank (not shown).
As is apparent from the above description, each of the aforementioned conventional liquid fuel shut-off valves is constructed such that the valve disc 5 is displaceable relative to the float 4. In order to prevent the valve disc 5 from failing to be lowered by gravity acting on both the valve disc 5 and the float 4, resulting in the fuel vapor outflow hole 3 remaining closed by the valve disc 5 when the liquid fuel has been returned from the valve housing 1 to the fuel tank, the valve disc 5 is received separately from the float 4 in the valve housing 1 so as to enable the valve disc 5 to be vertically displaced relative to the float 4. Further, the through hole 6 is formed with a diameter smaller than that of the fuel vapor outflow hole 3, so that the intensity of force of the float 4 against the valve disc 5 produced by the fuel vapor pressure in the fuel tank is smaller than that of the force of the valve disc 5 against the annular lower end edge of the fuel vapor outflow hole 3 produced by the fuel vapor pressure via the float 4. With such construction, the valve disc 5 is opened in two steps when the liquid fuel returns from the valve housing 1 to the fuel tank. However, to assure that the upper surface of the float 4 can reliably separate from the lower end of the through hole 6 (in the case as shown in FIG. 6) or the semispherical upper part of the rod-shaped protuberance 42 can reliably separate from the lower end of the through hole 6 (in the case as shown in FIG. 7), it is unavoidably required that the weight of float 4 be in excess of a predetermined value. Thus, it is practically difficult to design and construct the float 4 with smaller dimensions and thus it is practically difficult to design and construct the shut-off valve itself with smaller dimensions.